Photodetector and device employing the photodetector for converting an optical signal into an electrical signal

ABSTRACT

An anti-reflective coating having a composite layer of silicon nitride and silicon dioxide may be formed over the entire photosensitive region of the photodetector to minimize the amount of reflection. The composite layer comprises a silicon nitride layer and a dielectric layer contiguous to the silicon nitride layer. The anti-reflective coating may be formed in a CMOS process for fabricating the PN junction in the photodiode and CMOS devices for amplifying the photodetector signal, where the polysilicon gate layer is used as a etch stop. The P+ or N+ material in the PN junction of the photodiode has a distributed design where two portions of the region are separated by a distance in the range of Xd to 2Xd, where Xd is the one-sided junction depletion width, to enhance the electric field and to reduce the distance traveled by the carriers for enhancing bandwidth. A heavily doped region of the opposite type may be added between the two portions to further enhance the electric field. A mask is used to shield a portion of the substrate in which the photodetector region has been or is to be formed when other portions of the substrate region are implanted with a dopant to adjust at least one of the threshold voltages of the other portions. The mask prevents the photodetector region from being affected by such implant.

This application is a divisional application of U.S. patent applicationSer. No. 09/234,015, filed Jan. 19, 1999 now U.S. Pat. No. 6,218,719,which is a continuation-in-part of U.S. patent application Ser. No.09/156,872, filed Sep. 18, 1998, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates in general to photodetectors and, in particular,to an improved photodetector and a device employing the photodetectorfor converting an optical signal into an electrical signal.

CD-ROM and DVD-ROM drives have become increasingly important and popularfor use with personal computers and amusement game machines. CD-ROM andDVD-ROM drives typically employ optical pickup usually comprising twotracking photodetectors and four high speed split photodetectors, suchas that explained and shown in the article “High Speed SI-OEIC (OPIC)For Optical Pickup,” by Takimoto et al., IEEE Transactions on ConsumerElectronics, Vol. 44, No. 1, February 1998, pages 137-142.

As noted by the Takimoto article, CD-ROM and DVD-ROM drives areprogressing in terms of system compactness and performance. But when thesize of these drives are reduced, cross-talk has become an issue due tothe close spacing between the components of the drive. To reduce theinfluence of external noise from the motor and other electricalcircuits, the photodiode used in optical pickup has been integrated withthe preamplifier circuit that is used to amplify the output of thephotodetector.

CD-ROM and DVD-ROM drives have increasingly been used for reading videodata, such as data for motion pictures. For such applications and forimproved performance in other applications, it is important for theCD-ROM and DVD-ROM drives to have wide bandwidths.

Optical pickup components currently employed in CD-ROM and DVD-ROMdrives are typically bipolar devices. While bipolar devices may haveacceptable performance for such applications, with the intensecompetition in the consumer electronics industry, it is desirable toprovide alternative designs that are cheaper than the current opticalpickup designs.

None of the above-described conventional optical pickup devices forCD-ROM and DVD-ROM drives are entirely satisfactory. It is, therefore,desirable to provide an improved optical pickup and photodiode design toavoid some of the difficulties described above.

SUMMARY OF THE INVENTION

Applicant proposes an optical photodetector device implemented as CMOSdevices which are much cheaper than bipolar photodetector devices.Preferably, the CMOS processing circuit for amplifying the photodiodeoutput and the photodiode may be implemented in the same semiconductorsubstrate.

In another aspect of the invention, the P or N type semiconductormaterial that forms one side of the PN junction has at least twoportions that are spaced apart by not more than twice the one-sidedjunctioned depletion width in a configuration referred to herein as adistributed structure or configuration. By employing a PN junction ofsuch type where the semiconductor material forming one side of thejunction is so distributed, this has the effect of increasing both thedensity and amplitude of electric field in the depletion region of thephotodiode, thereby reducing drift time of carriers in the depletionregion. The responsivity can be further increased by preferablyemploying a highly doped semiconductor region between the two portionsof the semiconductor material that forms one side of the junction.

When the photodiode with a distributed configuration of semiconductormaterial as one side of the PN junction is employed in a CD-ROM or aDVD-ROM drive, it is preferable for the two spaced apart portions ofsuch material to be spaced apart by a spacing in the range of 5 to 15microns.

To further enhance the responsivity of the photodiode, ananti-reflective filter is employed over the entire photosensitive regionon the surface of a semiconductor substrate. The filter includes a firstlayer of silicon nitride and a second dielectric layer contiguous withthe first layer.

In fabricating CMOS devices, a threshold Vth implantation (hereinafterreferred to as “Vth implant” or “Vth implantation”) is performed toadjust the threshold voltage(s) of the CMOS devices. Where the CMOSdevices and the photodetector are fabricated on the same substrate, amask is preferably provided to shield the portion of the substrate inwhich the photodetector region has been or is to be formed during theimplantation, and the Vth implantation is performed only on the portionof the substrate that is not shielded by the mask, so that suchimplantation does not affect the photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of an integrated circuit of aphotodetector and CMOS processing circuit, photodetector mask andphotoresist to illustrate the circuit and a process for forming ananti-reflective coating for the photodetector and its processingcircuit.

FIGS. 1B-1D are cross-sectional views of an integrated circuit resultingfrom further processing steps on the circuit of FIG. 1A to illustratethe steps in a process for forming the anti-reflective coating.

FIG. 2A is a graphical illustration of the reflectance of a siliconnitride and silicon dioxide composite layer coating for 653 and 790nanometers wavelengths, where the thickness of the silicon nitride layeris about 700 nanometers, to illustrate the reflectance as a function ofthe thickness of the silicon dioxide layer.

FIG. 2B is a graphical illustration of the reflectance of a compositesilicon nitride and silicon dioxide layer anti-reflective coating for653 and 790 nanometers wavelengths, where the thickness of the silicondioxide is about 255 nanometers thick, to illustrate the reflectance asa function of the thickness of the silicon nitride layer.

FIG. 2C is a graphical illustration of the reflectance of a photodiodeintegrated circuit with and without the anti-reflective coating of thisinvention as a function of the total thickness of the silicon nitride,silicon dioxide and package layers placed on top of the anti-reflectivecoating.

FIG. 3A is a schematic view of six photodetector elements for CD-ROM andDVD-ROM optical pickup applications.

FIG. 3B is a schematic view of an optical pickup configuration readingdata from an optical media such as a disk to illustrate the invention.

FIGS. 4A-4D are cross-sectional views of a portion of a photodetectorelement suitable for use in any one of the photodetectors of FIG. 3A toillustrate four different embodiments of the invention employing a PNjunction structure between two types of semiconductor material, with onetype of material having a distributed structure to illustrate thepreferred embodiments of the invention.

FIG. 4E is a cross-sectional view of a portion of a conventionalphotodetector.

FIG. 5A is a graphical plot of the electric field vector in a directionnormal to the surface of the photodetector integrated circuit toillustrate the effect of the distributed N+ cathodes in a PN junctionstructure of this invention on the electric field in the photodetector.

FIG. 5B is a graphical plot of the electric field profile similar innature to that of FIG. 5A, but with a different junction geometry.

FIGS. 6A and 6B are graphical plots of the electric field profilesimilar in nature to FIG. 5A at two different reverse bias voltages. Thesemiconductor structure illustrated in FIGS. 6A, 6B differs from that ofFIG. 5A in that, in addition to the distributed PN junction structure ofFIG. 5A, a P+ region is sandwiched between the two N+ cathodes alsopresent in the structure of FIG. 5A, so that FIGS. 6A, 6B illustrateeffects of the electric field of the revised combined structure.

FIG. 6C is a graphical plot of the electric field profile similar innature to FIG. 5B, but where the effects of an additional P+ regionsandwiched between N+ cathodes are also shown.

FIG. 7A is a graphical plot of the electric field profile obtained witha conventional photodetector design.

FIG. 7B is a graphical plot of the electric field profile of aconventional photodetector design with a reverse bias voltage differentfrom that of FIG. 7A.

FIG. 8A is a cross-sectional view of a quad detector suitable for use inthe photodetector of FIG. 3A to illustrate an embodiment of theinvention.

FIG. 8B is a cross-sectional view of a portion of the quad detector ofFIG. 8A.

FIG. 9A is a cross-sectional view of one of the four detectors in a quaddetector suitable for use for the photodetector of FIG. 3A to illustrateanother embodiment of the invention.

FIG. 9B is a cross-sectional view of a portion of the detector of FIG.9A.

FIG. 10A is a cross-sectional view of a photodetector suitable for usefor one of the detectors in the quad detector of FIG. 3A to illustrateyet another embodiment of the invention.

FIG. 10B is a cross-sectional view of a portion of the detector of FIG.10A.

FIG. 11A is a cross-sectional view of a quad detector suitable for usein the photodetector of FIG. 3A.

FIG. 11B is a cross-sectional view of a portion of the detector of FIG.11A.

FIGS. 12-19 are cross-sectional views of semiconductor substrates toshow the processing steps for fabricating CMOS devices and aphotodetector in the same semiconductor substrate, where a Vthimplantation is performed on the portion of the substrate for the CMOSdevices but shielded from the region of the substrate for thephotodetector to illustrate the invention.

FIGS. 20-24, 25A-25C and 26A-26D are cross-sectional views ofsemiconductor substrates after the device from FIG. 19 have been furtherprocessed to show the different processing steps for fabricating asecond polysilicon layer to form resistors and capacitors and to use asecond polysilicon layer as an etch stop during the fabrication of anantireflective (“AR”) coating to illustrate the invention.

For simplicity in description, identical components in this applicationare identified by the same numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1A, a photodetector integrated circuit 20A includes aphotodetector 22 and CMOS devices 24 which form a processing circuit forprocessing (e.g. amplifying) the output of photodetector 22 toillustrate the invention. In contrast to conventional devices whichemploy bipolar or a combination of bipolar and CMOS devices to processthe output of the photodetector, in the preferred embodiment of FIG. 1A,only CMOS devices are used for processing the photodetector output. CMOSdevices consume less power and are cheaper to make than bipolar devices.The entire integrated circuit 20A (and circuits 20B, 20C, 20D of FIGS.1B-1D) may be made using a CMOS process so that the integrated circuit20A is inexpensive to make.

FIGS. 1A-1D are cross-sectional views of an integrated circuit atdifferent stages of processing for forming a composite anti-reflectivecoating of the desired composition and thicknesses to illustrate oneaspect of the invention. As shown in FIG. 1A, since the CMOS processtypically form a layer of polysilicon to be the gate of CMOS devices,such a layer is also formed over the photodetector 22.

In forming the CMOS devices, the polysilicon layer is separated from thetop surface of the N-epitaxial layer by a thin layer of silicon dioxidehaving a well controlled thickness. Therefore, the same silicon dioxideof controlled thickness also separates the polysilicon layer 26 from thephotodetector 22. On top of the polysilicon layer is an interleveldielectric layer 28 and an intermetal dielectric layer 30 formed whenthe CMOS devices 24 are formed. Therefore, as a first step infabricating the anti-reflective coating, a layer of photoresist 32 isformed on top of the intermetal dielectric layer 30 by means of PD mask34. The intermetal dielectric and interlevel dielectric layers 30, 28are etched down to the polysilicon layer 26, using the polysilicon layeras a mask or etch stop. Using the polysilicon layer as an etch stoppreserves the integrity of the surface interface between the wellcontrolled silicon dioxide layer underneath the polysilicon layer andthe photodetector 22. The resulting structure 20B is shown in FIG. 1B.The polysilicon layer 26 is then removed, leaving the very thin and wellcontrolled silicon dioxide layer above the photodetector 22 and the N−epitaxial layer 21. Such structure 20C is shown in FIG. 1C. A layer ofsilicon dioxide 42 is then deposited on structure 20C and then a siliconnitride layer 44 is deposited on top of the silicon dioxide layer toform the structure 20D in FIG. 1D. The silicon dioxide layer, which wasoriginally between the photodetector 22 and the polysilicon layer 26 andwhich was left after the polysilicon layer has been removed, bonds wellto the silicon dioxide layer deposited thereon.

The silicon dioxide layer 42 reduces leakage current at thephotodetector 22. By forming a uniform layer of silicon dioxide over theentire active region of the photodiode serving as the photodetector, thethickness of the silicon dioxide can be well controlled. This isimportant to minimize the amount of reflection of the light. Thus, whena light signal is incident on the silicon nitride layer immediatelyabove the photodetector 22, the light encounters a first interfacebetween the medium (e.g. package layer 46) above the silicon nitride andthe silicon nitride layer, the interface between the silicon nitridelayer and the silicon dioxide layer, and lastly between the silicondioxide layer and the photodetector 22. The thicknesses of the siliconnitride layer and of the silicon dioxide layer are selected to maximizedestructive interference of the light reflected from such interfaces,thereby minimizing the amount of light reflected from the structure 20Dand maximizing the amount of light that is transmitted through suchinterfaces to the photodetector 22. In order to minimize the amount ofreflection and to maximize the amount of light transmitted through thedifferent interfaces, it is important for the thickness of the silicondioxide layer and that of the silicon nitride layer to be wellcontrolled. As noted above, the formation of a uniform silicon dioxidelayer enables the thickness of the dioxide layer to be well controlled.Furthermore, the silicon dioxide and the silicon nitride layers arecompatible with the fabrication of CMOS devices 24 and may, therefore,be advantageously used for the purpose described.

Structure 20D is normally encapsulated by a packaging material, such asa transparent package layer 46 shown in FIG. 1D. Preferably, the packagelayer 46 has an index of refraction in the range of about 1.52 to 1.57to minimize the amount of reflection and to maximize the amount of lighttransmitted to the photodetector 22.

The structure 20D may be used for converting a light signal into anelectrical signal in a CD-ROM or DVD-ROM which employ light at twodifferent wavelengths: 653 and 790 nanometers. For this reason, it isdesirable for the anti-reflective composite coating, comprising layers42 and 44, to be optimized for optical signal to electrical signalconversion at the operating wavelengths of CD-ROM and DVD-ROM, that isat the wavelengths of 653 and 790 nanometers. FIG. 2A is a graphicalplot of the reflectance of the composite anti-reflective coating 42 and44, where the silicon nitride layer has a thickness of about 700nanometers and the reflectance of the composite coating is shown as afunction of the silicon dioxide thickness for both wavelengths. As canbe seen from FIG. 2A, the range of thicknesses of the silicon dioxidelayer within which the reflectance of the composite coating 42, 44 isminimized for both wavelengths is within the range of about 262±22nanometers (that is, range of about 240 to 285 nanometers). Morepreferably, such range is 262±15 nanometers. Thus, FIG. 2A is obtainedby performing simulation of a structure similar to structure 20D havingcomposite layers 42, 44 by keeping the thickness of the silicon nitrideunchanged at about 700 nanometers but varying the thickness of thesilicon dioxide layer.

The same can be done by keeping substantially constant the thickness ofsilicon dioxide and varying the thickness of silicon nitride as shown inFIG. 2B, where the thickness of silicon dioxide is fixed at about 255nanometers. As seen in FIG. 2B, the thickness of the silicon nitridelayer is preferably within the range of 700±30 nanometers in order tominimize the reflectance from the composite coating. More preferably,the range of thickness of the silicon nitride layer is in the range of700±20 nanometers. As shown in FIGS. 2A, 2B, if the thicknesses of thelayers 42, 44 are optimized, the reflectance at both 653 and 790nanometers approach zero. The anti-reflective coatings thereforefunction as an anti-reflective filter which substantially filters outradiation at these two wavelengths.

While the antireflective coating comprising layers 42, 44 is describedto comprise a silicon dioxide layer and a silicon nitride layer, twolayers made of silicon material other than silicon nitride and silicondioxide may be used instead and are within the scope of the invention.Such materials may include SOG-oxynitride, silicon-oxynitride andpolyimide film. In other words, the two layers 42, 44 may be chosen fromthe group of materials including silicon dioxide, silicon nitride,SOG-oxynitride, silicon-oxynitride and polyimide film.

The thickness of the package layer 46 may also be optimized in a similarmanner. As shown in FIG. 2C, where the total thickness of the compositelayers 42, 44, 46 has been optimized at a value (e.g. 480,500nanometers) that yields minimum reflectance, the reflectance can beminimized at both 653 and 790 nanometers wavelengths. In FIG. 2C, thetwo sinusoidal curves in solid lines illustrate the reflectances of thephotodetector integrated circuit (PDIC) without antireflective coatingat the two wavelengths 650 and 790 nanometers. These are labelled at thebottom of FIG. 2C as “NEB” (that is, no AR coating etchback). The twocurves in dotted lines illustrate the reflectances of the photodetectorintegrated circuit (PDIC) with antireflective coating and are labelledas dotted lines at the bottom of FIG. 2C as “WEB” (with AR coatingetchback).

FIG. 3A is a schematic diagram of six photodiodes suitable for use inCD-ROM and DVD-ROM optical pickup applications. As shown in FIG. 3A, thesix photodetectors A, B, C, D, E, F, are located in the optical pickuphead in the CD-ROM or DVD-ROM. FIG. 3B is a schematic view of an opticalpickup configuration reading data from an optical media such as a diskto illustrate the invention. As shown in FIG. 3B, the optical pickup 50suitable for use in CD-ROM and DVD-ROM applications contains aphotodetector device such as device 20D shown in FIG. 1D, objectivelenses 52, a quarter-wave plate 54, a polarizing prism 56 and a laser58. Laser 58 supplies a laser beam 58 a which is collimated by lens 52to polarizing prism 56 and is altered in polarization by quarter-waveplate 54 and focused again by another objective lens 52 to the opticalmedia such as a compact disk 59. The reflection from media 59 iscollimated by objective lens 52 and altered again in polarization byplate 54 and reflected by polarization prism 56 and focused by anotherlens 52 to the photodetector device 20D. As shown in FIG. 3B, opticalmedia 59 has tracks 59 a thereon.

The two detectors E1/E2 and F1/F2 are for tracking purposes, that is, toensure that the optical pickup head is in the right position relative tothe data on a CD-ROM disk or DVD-ROM disk in order to read the datarecorded thereon. Typically, the CD-ROM and DVD-ROM disks 59 have tracks59 a thereon and the two detectors E1/E2, F1/F2 are for ensuring thatdetectors A-D are in the right positions for reading the data relativeto the tracks on the disk. If the detectors A-D are not in the rightpositions, tracking of the optical pickup is adjusted automatically by aservo system until they are in the right positions. The four detectorsA, B, C, D are for reading the data; since it is arranged at a cornersof a square, the four detectors are collectively known as a quaddetector.

According to the CD-ROM and DVD-ROM specifications most commonly used,each of the four detectors A-D occupies a 50×50 micron area and the fourdetectors are separated by a spacing of about 5 or 10 microns both inthe horizontal and vertical directions as shown in FIG. 3A.

In the conventional photodiode design as shown in FIG. 4E, thephotodiode comprises a N+ region implanted in a P− epitaxial layer, sothat a PN junction is formed between the N+ region and the P− region.The two regions form a junction which is reverse biased by a voltagesupply VA. When the junction is so reverse biased, a depletion region 70is formed largely in the P− region surrounding the N+ region as shown inFIG. 4E, where the depletion region 70 has a width Xd known also as theone-sided junction depletion width. When light impinges on the PNjunction 52 as shown in FIG. 4E, electron-hole pairs are formed in thedepletion region 70 and the electric field present in the depletionregion causes drift of the electrons and holes, or carriers, to the P+substrate and the N+ region, and then to the electrical contacts of thereverse biased voltage supply to the P+ substrate and the N+ region. Thepercentage of electrons and holes so formed that are able to travel tothe P+ substrate and N+ region within the shortest time determines thebandwidth and responsivity of the photodiode. The higher the percentage,the greater is the responsivity. As noted above, it is desirable toincrease such percentage so that the photodiode 52 will have a highresponsivity.

The Applicant has recognized that two factors affect the percentage ofelectrons and holes that will be collected and transmitted as currentthrough the contact points of the photodiode to an external processingcircuit. The first factor involves the speed by which the carriers moveor drift to these contact points, where such speed varies directly withthe electric field strength over the paths of the carriers. In FIG. 4E,for example, the electrons that are formed will have to drift towardsthe N+ region and the holes will have the drift to the P+ substrate, andthe electric field strength along such paths will affect the bandwidth.It should be noted that these paths are largely along directions normalto the surface of the integrated circuit. If the surface of theintegrated circuit is in the XZ plane, then the paths are largelyparallel to the Y-axis, so that the electric field that largelydetermines the drift velocity of the carriers is the electric fieldalong the Y-axis.

The second factor affecting the percentage of carriers that will becollected and transmitted as current is the distance over which thecarrier drift should occur. Thus, by increasing electric field strengthand by reducing the distance along the paths that the electrons andholes must travel to reach such contact points, the bandwidth of thephotodiode can be greatly enhanced.

FIGS. 4A-4D are cross-sectional views of four different embodiments of aPN junction where either the P or N type semiconductor material has adistributed configuration, to both reduce the distance traveled by theelectrons and holes and to increase the electric field strength in theareas affecting the drift of the electrons and holes.

The photodiode 60 in FIG. 4A is formed by first growing a N− epitaxiallayer on a N+ substrate, doping at two separate areas of the epitaxiallayer so that two P+ regions 62 a, 62 b are formed that are adjacent toeach other. Another N− epitaxial layer is grown on top of the structureso formed so that a buried PN junction results. The two P+ regions 62 a,62 b are connected by a P+ connecting portion or an electrical conductorto form a single P+ region 62 so that a single PN junction is formedbetween the P+ region and the N− epitaxial layer. Thus, when a reversebias is applied by a voltage source VA across the P+ and N− junction, adepletion region is formed in the N− epitaxial layer surrounding the P+region as shown in FIG. 4A. The one-sided junction depletion width Xd isproportional to the square root of the amplitude of the voltage appliedby the voltage source VA.

To increase the electric field in the depletion region and therefore thedrift velocity of the carriers (electrons and holes), the distancebetween the two portions 62 a, 62 b of the P+ region is preferably notmore than two times Xd. Where the spacing between the two portions 62 a,62 b is within such range, the distances traveled by at least some ofthe carriers to the contact point are also reduced. Therefore, thedistributed nature of the semiconductor material in region P+ increasesthe bandwidth and the responsivity of the photodiode 60 relative to theconventional photodiode design 52.

FIG. 4B is a cross-sectional view of a PN junction of a photodiode toillustrate another embodiment of the invention. In reference to FIGS.4A, 4B, the photodiode 60′ is different from photodiode 60 of FIG. 4A inthat photodiode 60′ is a surface junction whereas that of photodiode 60is a buried junction. Thus, the surface junction 60′ has no additionalN− epitaxial layer grown on top of the P+ regions or on the original N−epitaxial layer, so that the contact between the voltage source and theP+ region may be formed directly. Again, the spacing between the twoportions 62 a′, 62 b′ of the single P+ region 62′ is not more than twicethe one-sided junction depletion width. Preferably, the thickness of theN− epitaxial layer is in the range of about 10 to 15 microns to maximizethe responsivity at 653 and 790 nm.

FIGS. 4C and 4D are similar to those of FIGS. 4A and 4B, respectively,except that the photodiodes in FIGS. 4C and 4D are formed starting witha P+ substrate, growing a P− epitaxial layer on the substrate, and byimplanting dopants to form N+ doped regions in the epitaxial layer. Thephotodiode shown in FIG. 4C is a buried junction whereas that in FIG. 4Dis a surface junction. In FIG. 4D, preferably, the thickness of the P−epitaxial layer is in the range of about 8 to 10 microns to maximize theresponsivity at 653 and 790 nm.

The two portions 62 a, 62 b or 62 a′, 62 b′ of the integral P+ regionsin FIGS. 4A and 4B and the corresponding two portions of the integral N+regions in FIGS. 4C and 4D form two capacitive plates whose capacitanceis inversely proportional to the distance or spacing between them. Sincejunction capacitive loading will limit the bandwidth response of thephotodetector, it is undesirable for such spacing to be too small.Therefore, in the preferred embodiment, it is preferable for suchspacing to be not less than Xd, the one-sided junction depletion width.

FIG. 5A is a graphical plot of an electric field profile of a structuresuch as that in FIGS. 4C and 4D that are reserve biased at 2.5 volts,where the amplitude of the electric field vector shown is the amplitudeof the field in a direction perpendicular to the substrate. In thenotation of FIG. 5A, the P+ substrate is in the XZ plane where the twoN+ regions are spaced apart along the X axis. In the notation in FIG.5A, “PSNSNSP=to 1-5-5-11-5-5-1” indicates that, along the X axis, goingfrom left to right in the figure, one encounters a 1 micron P regionseparated by a 5 micron spacing to the next semiconductor region whichis an N region of 5 microns wide which is separated from the next Nregion of 5 microns wide by a spacing which is 11 microns, where suchnext N region is separated by 5 microns from the next semiconductor Pregion of 1 micron in width, with dimensions all along the X axis.

FIG. 7A is a graphical plot of the electric field profile of aconventional photodiode structure such as that in FIG. 4E shown with thesame convention as FIG. 5A. The PN junction illustrated in FIG. 7A isalso reverse biased at 2.5 volts. In reference to FIGS. 5A and 7A, itwill be seen that the electric field has significant strength in a muchhigher percentage of the space around the PN junction in FIG. 5Acompared to that in FIG. 7A.

FIG. 5B illustrates the effects on the electric field profile by causingthe two distributed N+ portions of the single N+ region to be muchcloser together than that shown in FIG. 5A. As shown in FIG. 5B, the twoportions are at the spacing of 5 microns apart, so that the electricfield in the region between the two portions is much more intensecompared to that shown in FIG. 5A. For both profiles in FIGS. 5A and 5B,a much higher percentage of the space at or around the PN junctions isat high electric field strengths compared to that in FIG. 7A. Thejunction in FIG. 5B is reverse biased at 1.4 volts. The electric fieldprofile resulting from a conventional photodiode design such as that inFIG. 4E reverse biased at 1.4 volts as shown in FIG. 7B.

The Applicant also recognized that, by including a heavily doped regionbetween the two portions of the distributed semiconductor materialforming one side of the junction, the electric field amplitude can befurther enhanced, such as by adding a heavily doped P+ region betweenthe two N+ portions in the structures in FIGS. 4C, 4D, or by adding aheavily doped P+ region between the two N+ portions in the structures inFIGS. 4A, 4B. This is illustrated in FIGS. 6A-6C. In the structureillustrated in FIG. 6A, a P+ region having a width of 1 micron along theX axis is included half way between the two N+ regions, where the P+region is spaced 5 microns from each of the two N+ portions. In otherwords, the PN junction illustrated in FIG. 6A is the same as thejunction in FIG. 5A, except that an additional P+ region of 1 micron isadded midway between the two N+ portions. As compared to the electricfield profile in FIG. 5A, the electric field profile in FIG. 6A has amuch higher electric field amplitude in the region between the two N+portions. The additional P+ region of 1 micron is biased (not shown) atthe same voltage as the P− epitaxial layer and the P+ substrate in theconfigurations of FIGS. 4C, 4D. Similarly, where an additional N+ regionis added midway between the two P+ portions in the configurations ofFIGS. 4A, 4B, the additional N+ region is biased (not shown) at the samevoltage as the N− epitaxial layer and the N+ substrate.

In FIG. 6A, the PN junction is reserved biased at 2.5 volts. A similarelectric field profile for the same junction as that illustrated in FIG.6A but reverse biased at 1.4 volts instead of 2.5 volts is illustratedin FIG. 6B. FIG. 6C is a graphical illustration of the electric fieldprofile resulting from reverse biasing a PN junction similar to thatillustrated in FIG. 5B, but where an additional P+ region 3 microns widealong the X direction is included between the two N+ portions. As can beseen from a comparison between FIGS. 6C and 5B, the additional heavilydoped region between the two portions further enhances the amplitude ofthe electric field around the PN junction.

FIG. 8A is a cross-sectional view of a quad detector comprisingdetectors A, B, C, D to illustrate one embodiment of the invention. Asshown in FIG. 8A, each of the four detectors A-D comprises five strips102 of N+ regions connected at one end by a metal contact 106. Eventhough the five strips are not connected together by the same N+material, the fact that they are connected together by metal means thatthe five strips will be at the same electrical potential and thereforefunction as a common node in the semiconductor region in the PN junctionin such photodiode. The five strips of N+ material 102 are formed in theP− epitaxial layer, where between each pair of strips 102 is a narrowstrip of P+ material 104 for enhancing the electric field between thestrips. To facilitate the integrated circuit design, a single cellelement may be laid out such as that shown in FIG. 8B and then repeatedfive times for each of the four photodetectors A-D. Of course, it isalso possible to employ instead five strips of P+ material are formed inan N− epitaxial layer, where between each pair of strips is a narrowstrip of N+ material for enhancing the electric field between thestrips.

As shown in FIG. 8A, for CD-ROM and DVD-ROM applications, each of thefour photodetectors A-D is 50×50 microns square. Since the five stripsof N+ (or P+) material 102 are connected by metal at one end, the fivestrips may be regarded as a single N+ (or P+) region having fivedistributed portions. Another aspect of the invention is based on therecognition that, for CD-ROM and DVD-ROM applications, any two portionsin the distributed structure of one type of semiconductor materialforming one side of the PN junction in the photodiode are spaced apartby a spacing in the range of about 5 to 15 microns. When the spacingbetween adjacent portions of the single region is in such range, it islikely that the electric field strength is optimized.

FIG. 9A is a cross-sectional view of a distributed structure for one ofthe four detectors A-D in FIG. 3A to illustrate another embodiment ofthe invention. As shown in FIG. 9A, the distributed structure maycomprise two sets of three cross-shaped elements 112, where each set ofthree elements are connected together and to a common metal contact (notshown) to form a single N+ region. The two sets are separated by adistance less than twice the one-sided junction depletion width and by aspacing in the range of 5 to 15 microns. A heavily doped P+ region 114may be added between the two sets to further enhance the electric fieldstrength in the space between the two sets of N+ portions. FIG. 9B is aschematic view of a single cell element which may be repeated six timesfor the design of the photodetector of FIG. 9A.

FIG. 10A is a cross-sectional view of a photodetector which may be usedfor any one of the four detectors A-D of FIG. 3A. As shown in FIG. 10A,the N+ region comprises six circular or cylindrical portions 122connected together by means of abutting N+ connecting portions 122 a andat one end to an N+ contact 126. Again, a single cell design omittingthe connecting portion 122 a is shown in FIG. 10B. As before, P+ strips124 may be included to enhance the electric field strength.

FIG. 11 is a cross-sectional view of a quad detector (A, B, C, D) ofFIG. 3A to illustrate yet another embodiment of the invention. As shownin FIG. 11A, each of the four detectors A-D comprises a single N+ regionwith ten fingers; five at the top and five at the bottom, where eachpair of adjacent fingers are separated by a spacing which is in therange of Xd to 2Xd in the preferred embodiment, and preferably in therange of about 5 to 15 microns. Between each pair of adjacent fingers ispreferably a P+ region to enhance the electric field between thefingers. FIG. 11B is a schematic diagram of a single cell which may berepeated five times in each detector for the design of the quaddetector. In each of the designs of FIGS. 8A-11B, the N and the P typematerials may be reversed in their roles, so that P+ and N+ regions areformed in an N− epitaxial layer which has been grown from an N+substrate.

FIGS. 12-19 are cross-sectional views of semiconductor substrates toshow the processing steps for fabricating CMOS devices and aphotodetector in the same semiconductor substrate, where a Vthimplantation is performed on the portion of the substrate for the CMOSdevices but shielded from the region of the substrate for thephotodetector to illustrate the invention. FIG. 12 is a cross sectionalview of a semiconductor substrate 200 comprising an N+ substrate 202, alayer of N-epitaxial layer 204 and an oxide layer 206. As also shown inFIG. 12, a NWell mask and photoresist are employed for an NWell implant,FIG. 13 is a cross-sectional view of the substrate 200 and PWell maskand photoresist for a PWell implant, and the resulting semiconductorsubstrate 200′ with the resulting PWell and NWell implants is as shownin FIG. 14. As also shown in FIG. 14, a layer of silicon nitride 208 isformed on top of the silicon dioxide layer 206. An active mask to definethe nitride layer and a PWell mask for a P field implant are employed asshown in FIGS. 14 and 15 to define the silicon nitride layer and toperform a P field implant.

A photodiode active mask 220 is employed to shield a portion 204 a ofthe N-epitaxial layer 204, in which the photodiode is to be fabricated.The mask 220 shields only the region 204 a of the substrate 200″ wherethe photodiode is to be fabricated: the Vth implantation is performedpreferably on the remainder of the substrate in which CMOS devices areto be fabricated as shown in FIG. 16. The Vth implant is performed toadjust the threshold voltage of the CMOS devices. Such shielding of theregion 204 a will prevent the Vth implant from affecting thephotodetector that will be formed in the region. Thereafter, thephotodiode is fabricated in region 204 a by a PDP+ implant and PDPN+implant as shown in FIGS. 17 and 18. As shown in FIG. 19, a gatepolysilicon layer 230 is formed on the silicon dioxide layer 206, and apolysilicon layer 236 is also formed on the field oxide region 234.Preferably layers 230 and 236 are formed in the same processing step bymeans of a poly 1 mask 232. While in the preferred embodiment, thepolysilicon layer on the field oxide 234 is formed in the sameprocessing step as the gate oxide on the silicon dioxide layer 206, thisis not required; in other words, the polysilicon layer on the fieldoxide region 234 may be formed in a separate processing step employing adifferent mask from the gate or polysilicon layer on the silicon dioxidelayer 206. As described below, this polysilicon layer 236 forms thebottom plate of a capacitor 260.

Another silicon dioxide layer is deposited on top of the structure inFIG. 19 to form the poly 1/poly 2 oxide and gate oxide layer 242 asshown in FIG. 20; this layer 242 includes the layer 206 and theadditional silicon dioxide that is deposited on top of the structure inFIG. 19. As illustrated in FIGS. 20 and 21, a second polysilicon layer244 is then formed on top of the dioxide layer 242 and a poly 2 mask 246is employed in order to fabricate the poly 2 mask 248, the poly 2resister 250 and the top portion 252 of the capacitor 260 whichcomprises the polysilicon layer 236 formed as described above, thepolysilicon layer 252, and a layer of silicon dioxide between the layers236 and 252, where such dioxide layer is deposited as described above inreference to FIG. 20.

NLDD, PLDD implants are then performed as indicated in FIGS. 22 and 23and N+ S/D and P+ S/D implants are performed as indicated in FIGS. 24and 25A. Three metal layers M1, M2, M3 and vias 1 and 2 for contacts arethen formed as indicated in FIG. 25B and the antireflective (AR) coatingetchback is formed as indicated in FIG. 25C. The AR coating etchbackprocess indicated in FIG. 25C is illustrated in more detail in FIGS.26A-26D. Thus, after the metal and intermetal layers are formed, across-sectional view of the semiconductor substrate 200′″ is illustratedin FIG. 26A. Substrate 200′″ has a silicon dioxide layer 272 above thepolysilicon mask layer 248, and an additional oxide layer 274 above thedioxide layer 272 during the formation of the different metal layers. APD mask 276 and photoresist 278 are employed to etch layers 272, 274,using the polysilicon mask layer 248 as an etch stop, as illustrated inFIG. 26B. Shown more clearly in FIG. 20 is the poly 1/poly 2 silicondioxide layer 242 between the polysilicon layer 248 and region 204 awhere the photodiode is formed. The polysilicon layer 248 is removed,resulting in the structure shown in FIG. 26C. As noted above, since theinitial silicon dioxide layer 206 and the layer of silicon dioxidedeposited on top of layer 206 to form a combined silicon dioxide layer242 are both well controlled, the thickness of the silicon dioxide layer242 is well controlled. As illustrated in FIG. 26D, another silicondioxide layer is deposited on top of the entire structure of FIG. 26C.The silicon dioxide deposited on the dioxide layer 242 on top of thephotodetector region 204 a bonds well to layer 242 since the polysiliconlayer 248 preserves the integrity of the surface of layer 242 during theetch back process illustrated in FIG. 26B. The layer of silicon dioxidedeposited on the structure of FIG. 26C together with layer 242 form acombined layer 292 of silicon dioxide as shown in FIG. 26D. A layer ofsilicon nitride 284 is then deposited on top of the silicon dioxidelayer 282 to form the antireflective coating. In this manner, the totalthickness of the silicon dioxide layer 282 and of the silicon layer 284can be well-controlled to the desired thicknesses to minimize the amountof radiation reflected by the interfaces between layers 282, 284 andregion 204 a. The range of optimal thicknesses of the layers 282, 284and of any packaging layer on top of these layers are explained above.

As noted above, preferably the polysilicon layer forming the lower plate236 of the capacitor 260 is formed together with the gate polysiliconlayer 230 and the top polysilicon plate 252 of the capacitor 260 isformed together with the mask 248. Preferably, in reference to FIG. 16,the Vth implant process is performed during the formation of the CMOSdevices prior to the formation of the photodetector in region 204 a asillustrated in FIGS. 16-19.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents.

What is claimed is:
 1. A method for making a photodetector comprising:providing a structure having a photosensitive region on or in asemiconductor substrate, a masking layer over said region and a materialover said masking layer; removing from said structure the material oversaid masking layer using said masking layer as a mask; removing saidmasking layer; and forming a first layer of silicon nitride and a secondlayer of dielectric material contiguous with the first layer over saidregion, the second layer having a predetermined thickness.
 2. The methodof claim 1, wherein said forming forms the first and second layers oversubstantially the entire photosensitive region.
 3. The method of claim1, wherein said providing provides a structure having a polysiliconlayer over the substrate, a portion of said polysilicon layer being themasking layer.
 4. The method of claim 1, wherein said providing meansprovides a CMOS structure having a polysilicon gate layer over thesubstrate, and comprises forming said masking layer together with thepolysilicon gate layer.
 5. The method of claim 1, wherein said providingprovides a CMOS structure having a polysilicon gate layer and acapacitor over the substrate, said capacitor comprising a first and asecond polysilicon layer, and wherein said providing comprises formingsaid masking layer together with the first or the second polysiliconlayer or together with the polysilicon gate layer.
 6. The method ofclaim 5, wherein the polysilicon gate layer and said first polysiliconlayer are formed together, and said masking layer and said secondpolysilicon layer are formed therein.
 7. The method of claim 1, saidregion comprising a PN junction, wherein said forming forms the secondlayer in contact with the junction to reduce leakage current.
 8. Themethod of claim 1, wherein said forming forms said first and secondlayer so that said first layer has a thickness in the range of 7000±500Angstroms, and said second layer has a thickness in the range of 2400 to2850 Angstroms.
 9. The method of claim 1, wherein said forming forms thesecond layer so that the second layer includes a silicon dioxide layer.